In many modern televisions, a high voltage accelerator potential is developed by a flyback transformer for accelerating an electron beam in a cathode-ray tube. This potential is generally referred to as the high voltage and will be on the order of 25 to 30 kilovolts for most color televisions. Screen brightness is proportional to the electron beam intensity (electron beam current) which is generally proportional to the high voltage. Thus, to increase screen brightness as might be desired, one must increase the high voltage and beam current. However, a television receiver is designed to operate at a particular high voltage and beam current, and exceeding such design criteria may cause the production of X-radiation.
While the electron beam current is generally proportional to the high voltage, the actual relationship between these two parameters varies depending upon operating conditions of the receiver. Also, while the production of X-radiation is generally associated with excessive high voltage, in fact, the production of X-radiation is affected by a combination of both high voltage and beam current. In general, as the beam current is lowered, a greater high voltage is allowable without producing harmful amounts of X-radiation.
To protect against the production of X-radiation, protective circuitry is commonly used to disable the television when favorable conditions exist for the production of X-radiation. Early designs of such protective circuitry simply monitored the high voltage and disabled the television set whenever a threshold was exceeded. In this configuration, the design of the protection circuit must assume a worst case electron beam current and thus, the threshold must be set lower than necessary for most beam currents. This design requirement often resulted in an unnecessary disablement of the television set when the magnitude of the high voltage went high, but the electron beam current remained sufficiently low to negate any danger of X-radiation production.
Various protection circuits have been designed to avoid so called nuisance shutoffs during periods where low electron beam current negates the potential of producing X-radiation. For example, Willis in U.S. Pat. No. 4,126,816 discloses a circuit that monitors only high voltage during normal operating conditions. However, at a predetermined low beam current, a second circuit imposes a new bias on the protective circuit which raises the magnitude of the high voltage required to trigger protection of the television set. In essence, Willis discloses a high voltage monitor or protection circuit that operates in two modes: a low beam current mode and a high beam current mode. However, this monitor is not fully responsive to beam current since the monitor is decoupled from the beam current circuit except during periods of low beam current. If a malfunction occurred which caused a very high beam current with the magnitude of the high voltage remaining normal, this circuit would not respond to the extremely high beam current and would not trigger protection.
Another example of a high voltage protection circuit is disclosed in U.S. Pat. No. 4,287,535 to Vakil in which high voltage is monitored and a latch circuit is triggered when the high voltage magnitude is considered excessive. Vakil monitors a high voltage horizontal sweep transformer and connected to the transformer are a rectifier circuit and a Zener diode circuit. So long as the high voltage is below a predetermined threshold, the Zener diode decouples the latch circuit from the sweep transformer and rectifier circuit. However, when the high voltage exceeds the first threshold, the Zener diode breaks down and the latch circuit is coupled to the rectified voltage from the transformer. However, the latch circuit is not necessarily actuated by the aforementioned coupling. Instead, the current flow from the transformer through the rectifier and latch circuit must exceed a second threshold before the latch circuit will be actuated. This second threshold is variable depending upon input from a feedback circuit that is also connected to the high voltage sweep transformer and the cathode-ray tube anode. According to the patent, the second threshold is proportional in magnitude to the beam current. Thus, according to the patent, once the first and second thresholds have been exceeded, in that order, the latch circuit is permanently triggered. Regardless of the magnitude of the high voltage or the electron beam current, the latch will remain triggered until the television is turned off. Again, Vakil is not fully monitoring beam current since the high voltage monitor is decoupled from the latch except during periods when the magnitude of the high voltage exceeds the first threshold.
As a further example of a protection circuit, U.S. Pat. No. 4,213,166 describes a system in which beam current and high voltage are monitored to determine a condition for enabling operation of a high voltage shutdown circuit. A high voltage transformer winding generates a voltage proportional to the high voltage applied to the cathode ray tube. A second voltage is generated proportional to the beam current and is combined with the winding generated voltage. When the combined voltage exceeds a reference potential, the shutdown circuitry is enabled.
The foregoing techniques must remain stable during operation of the television receiver. Changes in voltages not specifically related to the generation of the X-ray conditions must not influence the operation of the protection circuit. Thus, variations in low voltage power supply voltage which occur over time must not alter the operation of the protective circuit.
The above circuits are but a few examples of numerous protection circuits for television receivers designed to protect against X-radiation. Many others exist but they are generally more complicated and expensive than necessary. Also, due to their complexity, they have an excessive number of failure modes. Finally, many of the protective circuits have been outmoded or made obsolete by changes in television design. For example, many televisions use a horizontal sweep transformer which was the signal source in the Vakil patent.